In order that this invention can be fully appreciated, the details of the construction of a boiling water nuclear fuel bundle will first be set forth. Thereafter the dynamics of fuel bundle operation will be partially disclosed. Emphasis will be placed on the functional interrelationship of a spacer within the upwardly flowing coolant within the nuclear fuel bundle located in the typical core of a nuclear reactor.
Fuel bundles in boiling water nuclear reactors include a matrix of vertical fuel rods, a lower tie plate for supporting the matrix of vertical fuel rods, and an upper tie plate for maintaining the matrix of fuel rods in vertical relation overlying the lower tie plate. A fuel channel is placed between the upper and lower tie plates around the fuel rods for defining a vertical flow path through the fuel bundle along the vertical length of the fuel bundle. The lower tie plate functions to permit the inflow of water at the bottom of the fuel bundle; the upper tie plate functions to permit the outflow of water and generated steam at the top of the fuel bundle.
The fuel bundles are elongate; typically they have a 8 by 8 inch square section and are about 160 inches in length. The contained fuel rods within the fuel bundle are in matrices on the order of at least 7 by 7 with more modern fuel designs having densities in the fuel bundles exceeding 10 by 10. The result is that absent a system of restraint, the fuel rods would tend to come into wearing and abrading contact with one another during the operational life of the fuel bundle. Such wearing and abrading contact would be responsive to the effects of creep and the dynamics of fluid flow about the fuel rods. Creep is a differential growth of the metal of the fuel rod cladding which occurs in the pressurized radiation environment of the fuel bundle causing the fuel rods to bend within the fuel bundle with age. The dynamics of fluid flow about the fuel rods causes the fuel rods to vibrate towards and away from one another during reactor operation.
To maintain the fuel rods in their designed side-by-side relationship, so-called fuel rod spacers are employed. These fuel rod spacers surround each fuel rod individually at selected vertical intervals along the length of the fuel bundle. Typically, between 5 to 7 spacers are distributed vertically along the length of the fuel bundle. Each spacer individually surrounds each fuel rod and confines that fuel rod to its designed location within the matrix at the elevation of the spacer. In short, the distributed spacers prevent abrading contact between the fuel rods, and maintain uniform rod spacing.
The spacers must exist interior of the fuel bundles in the upwardly flowing fluid stream between the lower tie plate and the upper tie plate. The spacers have a disadvantage of adding resistance to this required upward fluid flow. This resistance to fluid flow, if not controlled can lead to undesirable consequences. These undesirable consequences include the limitation of the power output of the fuel bundle as well as contributing to local or core wide instabilities including thermal hydraulic and nuclear thermal hydraulic instabilities in the upper two phase region of the fuel bundle.
To reduce the phenomenon of pressure drop, it has been the tendency of the prior art to make the material of the spacers as thin as possible.
Spacers are made of two materials. One material is a spring steel sold under the designation Inconel. These spacers can be made extremely thin; unfortunately, the spring material, even when very thin, is a relative high absorber of neutrons.
A more desirable spacer construction includes the use of a metallic alloy with low neutron absorption known as Zircaloy. While this metal is not a strong as Inconel, it neutron absorbing cross section is much more favorable.
One of the most successful designs of Zircaloy spacers includes the use of so-called ferrules at each rod location. A ferrule is a round cylinder surrounding the fuel rod but spaced a distance from the fuel rod by stops and a spring biasing the fuel rod onto the ferrule stops. When the fuel rod is biased onto the stops interior of an individual ferrule, a precise spatial relationship is maintained between the outside of the fuel rod and the inside of the ferrule. It has been found that this design is very favorable to the heat exchanging fluid flow passing over the fuel rod at and above the fuel rod at the location where it is surrounded by the ferrule.
In order to minimize pressure drop in the coolant flow and minimize neutron absorption, the ferule wall thickness should be as small as possible. Unfortunately, the dissolving of hydrogen carried within the water coolant of the nuclear reactor prohibits the use of Zircaloy sheet less than 0.020 inches thick. When Zircaloy sheet of lesser thickness is utilized to construct a spacer, the Zircaloy after exposure to the hydrogen rich environment interior of the reactor becomes brittle. Unfortunately, this brittle state of the hydrogen is particularly aggravated when the reactor is cold. It is when the reactor is cold and individual fuel rods are manipulated into and out of the fuel bundles for inspection and replacement that the spacers undergo relatively great stress. Simply stated, Zircaloy when used always has a thickness of at least 0.020 inches.
In a square array of ferrules, each ferrule is surrounded by and contacts four adjacent ferrules. At each of these contact points there is a double thickness of Zircaloy. If this double thickness could be eliminated, the spacer pressure drop and neutron absorption could be reduced.
It has been known to try and avoid these double thickness of Zircaloy by utilizing an "egg crate" or grid design in the fabrication of spacers. In such a design, the strips of metal form respective right angle partitions between adjacent fuel rods. Only a single thickness of Zircaloy is disposed between each of the fuel rods.
Unfortunately, this type of "grid" design has its own short coming.
In the upper portion of the fuel bundle, the liquid coolant rises with increasing fractions of generated steam. Typically, the relatively slowly upwardly moving water concentrates adjacent the fuel rods; the higher velocity upwardly flowing steam concentrates in the spatial intervals between the fuel rods. That spatial interval having the largest unobstructed area for the rapidly moving upwardly flowing steam is found along the diagonal spatial intervals between a matrix of fuel rods aligned in regular row and column intervals. Unfortunately, and with an "egg crate" or grid type spacer, the cross over between the strips of sheet metal forming the spacer occurs within this spatial interval having the largest unobstructed area. Consequently, undue pressure drop results. For this reason, the ferrule construction of spacer has been preferred over the grid type construction despite the higher neutron absorption of the ferrule type spacers.